1. Field of the Invention
The present invention relates to apparatus for monitoring usage of multiple electrical devices.
2. Description of the Relevant Art
Monitoring of usage of electrical equipment is often desirable to ensure that servicing or replacement is carried out at the proper time, and also for warranty purposes. Defense equipment, whose reliability may be safety critical, is an important case in point. Elapsed time indicators (ETIs) which connect electrically to the electrical supply of a piece of equipment and which contain an elapsed time counter, typically driven from the equipment's electrical supply, are in themselves well known. They may have an integral display or may, as exemplified in Oxley (Development) Co. Ltd's European Patent Application 00302679.6, have an interface for interrogation by a separate unit. The ETI described in that patent application monitors a single power supply, the elapsed time counter being activated when power is down by the host device.
Such simple ETIs are not well suited where multiple separate electrical devices or electrical sub-systems all require monitoring. One example of such a situation, of particular relevance for present purposes, is provided by the communications apparatus of a military vehicle, which may comprise multiple different systems including UHF radio, HF radio, ancillary equipment etc each requiring independent usage monitoring. Multiple separate ETIs, each associated with respective systems, would not be appropriate.
Oxley (Development) Co. Ltd's UK patent GB 2142172 describes a system in which for each electrical device to be monitored there would be a respective non-volatile memory device and interface circuit, connected to the electrical device's and adapted to maintain its own cumulative record of host running time, but several such devices would be connected via a data bus to a common control unit having a display and user controls. Connection of the multiple separate monitoring devices could still prove problematic in some contexts.
The requirement of such ETIs for an electrical connection to the supply of each device being monitored can be a drawback, giving rise to its own concerns over reliability (in safety critical systems, the possibility that the ETI might itself compromise reliability of the system being maintained is desirably to be avoided) and creating potential installation problems, particularly where the usage monitoring system is to be “retro fitted” to an existing piece of equipment.